The present invention relates to a control apparatus for an internal combustion engine including a catalyst that purifies exhaust discharged from a plurality of cylinders, and a plurality of fuel injection valves respectively provided for the plurality of cylinders.
Japanese Laid-Open Patent Publication No. 2004-218541 describes a control apparatus that executes dither control. When a request for temperature increase of a catalytic apparatus is made, the air-fuel ratio in some cylinders is set to be richer than a stoichiometric air-fuel ratio and the air-fuel ratio in the remaining cylinders is set to be leaner than the stoichiometric air-fuel ratio by the dither control so that the air-fuel ratio of exhaust flowing into a catalyst equals a target air-fuel ratio.
In addition, an imbalance learning process is known in which when fuel injection valves are operated to equalize air-fuel ratios in a plurality of cylinders, the variation (imbalance) degree among injection amounts of the fuel injection valves of the plurality of cylinders is learned.
When the imbalance occurs, the air-fuel ratio of an air-fuel mixture that is combusted in each cylinder is deviated from a target value. Thus, the combustion controllability is declined in each cylinder. When dither control is executed, some cylinders are set to have an air-fuel ratio that is richer than a stoichiometric air-fuel ratio and some cylinders are set to have an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. Thus, the combustion state of each cylinder intentionally needs to be deviated from an optimum state. Therefore, when dither control is executed during an occurrence of an imbalance, the dither control may promote a decline in the combustion controllability attributable to the imbalance.